The dead bug is a good exercise to train the control of pelvic position with isometric ab contraction.
Lie on your back and crunch the abs to bring the tailbone and shoulders up off the floor. This crunched position needs to be maintained throughout the set—the point is to fight against the pelvis rotating forward as the legs extend and place tension on it.
The McGill curl-up crunch is a good combination of continuous ab tension like an isometric hold and minimal motion for the abs while maintaining neutral spine curvature, and as a way to practice and reinforce trunk bracing. It can be used for a low-demand ab exercise to reduce difficulty or to allow very high repetitions, and it can be used in training preparation to get the trunk activated without excessive fatigue.
The dead bug can be done with straight legs to increase difficulty. Small weights or bands can also be added.
The McGill curl-up should generally be done for 2-4 sets per leg of 3-5 breath cycles or 10-15 second holds. This is a good exercise for training preparation, or in addition to heavier, lower-rep or more difficult ab work in a given workout or in a circuit.